Method and apparatus for the thermal production of metal carbides and metals

ABSTRACT

A method and apparatus for the continuous production of metals and/or carbides by thermal reduction is described. In the process, a mixture of metal oxide and carbon is agglomerated, each piece of agglomerate encased in carbon and/or graphite, the carbon casing serves as a housing of the agglomerate during the reducing reaction and as an electrical resistance element for heating of the agglomerate, the encased agglomerates are coked and reduced to metal carbide in a tightly-packed form. Extraction of the metal from the carbide is effected with a halide of the same metal.

This is a division of application Ser. No. 211,054, filed Nov. 28, 1980,now U.S. Pat. No. 4,441,920, issued Apr. 10, 1984.

The present invention relates to a method and apparatus for the thermalproduction of a group of metal carbides and/or metals wherein a mixtureof metal oxide and carbon is reduced to metal carbide and the metal thenextracted from the metal carbide. Carbon or graphite is used toencapsulate the agglomerates of the metal oxide and carbon. The carboncapsule or jacket serves as a housing for the agglomerate during thereducing reaction. The amount of carbon required for reduction of themetal oxide and formation of the carbide is provided in the agglomerate.The group of metals to which the invention pertains include thecarbide-forming (generally considered hard-to-reduce) metallic elementsaluminum, boron, silicon, titanium, zirconium, tantalum, niobium,molybdenum, tungsten and uranium.

It is a particular object of the invention to produce aluminum in theform of a pure metal, in a two-stage process, via the intermediateproduct aluminum carbide, but to generally reduce the oxides of theother metals mentioned above only to the stage of stable carbides.

The thermal processes for producing the above-mentioned carbides andmetals generally differ quite substantially from one another and areusually tailored to the particular metal concerned. The method accordingto the present invention permits the production of these metal carbidesand at least partial extraction of the metals therefrom according to acommon principle. Using aluminum as an example, the basic problems ofcarbothermic reduction of metal oxides that are hard to reduce, as wellas details of the method according to the invention, will be describedin greater detail.

The known single stage methods for the thermal direct reduction ofaluminum oxide with carbon have thus far not been proven to be of valuein commercial practice due to chemical, physical and proceduralproblems. In particular, it has not been possible to develop acontinuous industrial process.

The chemical problems are primarily due to the fact that, in accordancewith thermodynamics, the reduction of aluminum oxide with carbon willfirst lead over the intermediate stage of oxycarbide to the formation ofaluminum carbide (Al₄ C₃). Aluminum carbide formation is basicallyfavored over aluminum formation. Through a reduced supply of carbonduring the reduction process, formation of aluminum carbide can besuppressed and the proportion of aluminum increased. However, with adecrease in the quantity of carbon present, the competing reactionleading to the formation of volatile aluminum suboxide (Al₂ O) isincreasingly favored. Thus, the increasing volatilization of thealuminum oxide used in the process must be reckoned with, if aluminumformation is to be increased in the known processes.

The course of reduction in the prior art processes is furthermorehindered because the preferentially formed aluminum carbide dissolvesboth in aluminum oxide and aluminum. Although the dissolved aluminumcarbide can be further converted with aluminum oxide to aluminum, underpractical conditions this reaction does not proceed exclusively in thedirection of aluminum.

Another aggravating circumstance is the relatively narrow temperaturerange of 2050 to 2150 degrees centigrade that must be maintained for thealuminum oxide reaction to achieve a favorable yield in aluminum.Beginning at about 2000 degrees centigrade, an undesirable vaporizationof the aluminum is noticed. For the formation of aluminum carbideaccording to the invention the preferred reduction temperatures liebetween 1950 and 2050 degrees centigrade. At these temperatures and witha sufficient carbon supply, the vaporization losses through aluminumsuboxide and aluminum can be kept to a minimum. The required narrowtemperature range of around 2000 degrees centigrade poses a serioustechnical problem for the reduction process for producing aluminumcarbide.

The physical difficulties of carbothermic aluminum production are thatthe molten aluminum oxide (melting point around 2050 degrees centigrade)is specifically heavier than the liquid aluminum. A molten mixture ofaluminum oxide, aluminum carbide and carbon (which mixture is present ingreater quantity than the tappable aluminum) collects at the bottom ofthe reducing furnace. Also, graphitized carbon which can form during thereduction process has about the same density as liquid aluminum. Anenrichment of aluminum carbide leads to hard-to-melt compositions (asaluminum carbide constitutes a solid phase) and with higherconcentrations in aluminum oxide or aluminum increases the meltingpoints of the binary mixtures.

The problems of a carbothermic aluminum oxide reduction shown here to beof a chemical and physical nature naturally lead to considerableprocedural difficulties. Because of the required high reductiontemperatures electric arc furnaces were principally used for industrialaluminum oxide reduction, as is customary in electrometallurgy. Only insmall-scale tests were directly or indirectly heated resistance furnacesused; the heating elements consisting of carbon or graphite. Neitherelectric arc furnaces nor other electrical heating systems used hithertohave proven successful for the carbothermic production of aluminum.

In addition to single-stage reduction of aluminum oxide to aluminum,two-stage processes have also been proposed wherein a mixture ofaluminum and aluminum carbide that can be tapped in the liquid stage isproduced first in an arc furnace, and then the aluminum separated fromthe Al--Al₄ C₃ melt by liquation in the presence of a flux or obtainedvia an aluminum subchloride distillation. According to another proposedmethod, the aluminum-containing melt products are mechanically preparedby hot milling and the aluminum then separated by straining. The controlof the reduction process in the arc furnace and the oxide-free tappingof the very hot and relatively light Al--Al₄ C₃ melt present enormousdifficulties.

A three-stage process for thermal aluminum production has also beenproposed in which aluminum carbide is produced as an intermediateproduct. With a limited amount of carbon the aluminum oxide is reducedto a gaseous mixture of Al₂ O, Al vapor and CO. The mixture of Al₂ O andAl vapor (distilled out of the reducing furnace) is then converted withan excess of carbon to aluminum carbide. In a third stage, the aluminumcarbide is decomposed to aluminum and carbon at temperatures of 2000degrees centigrade and pressures of 20 to 50 torr. The aluminum iscondensed as a liquid or solid. The drawbacks of this three-stageprocess lie in the fact that the thermal decomposition of aluminumcarbide under vacuum is extremely difficult to bring about. Additionaldifficulties complicate the complete reaction of Al₂ O with the carbonto form Al₄ C₃ and the reverse reaction of Al vapor with CO.

An overview of the known carbide processes of thermic aluminumproduction is given by E. Herrmann in the journal "Aluminium" 1961, No.4, pages 215-218.

To facilitate explanation, the present invention is described withreference to the production of aluminum and aluminum carbide. It shouldbe understood that this is a non-limiting example of the process of thepresent invention, which can be used to prepare other metals and theircarbides.

It is an object of the present invention to avoid the drawbacks of theprior art processes and to provide a method and apparatus for thecontinuous production of aluminum carbide and metallic aluminum.According to the invention, this object is achieved by agglomerating amixture of aluminum oxide and carbon, encasing each piece of agglomeratein carbon and/or graphite, coking and reducing the so agglomeratedbodies in tightly-packed form to yield aluminum carbide and extractingthe aluminum carbide with aluminum fluoride.

The first important step of the process according to the inventioninvolves the proper selection of charging material and its preparation.A fine-particle aluminum oxide is mixed with a carbon carrier of goodbinding property. The particulate aluminum oxide material preferred foruse in the invention is of the type commonly used for feeding theelectrolytic cell of a smelting operation. Preferably, the oxideparticles have an average particle size of about 50 micrometers and amaximum particle size of 180 micrometers (This means all of the oxideparticles pass an 80 mesh (Tyler) sieve.) As the carbon carrier, it ispossible to use carbon binders such as tars, pitches, resins orespecially the so-called extracts from solvent and pressure extractionof bituminous coals. These materials can be used alone or mixed withpowdered carbon or coke particles.

The green mixture of aluminum oxide and carbon binder is agglomeratedinto small briquettes. Agglomeration in the sense of the present textrefers to processes for making fine-particle materials into lumps, asdescribed in the journal "Chemie-Ingenieur-Technik" 51 (1979), No. 4,Pages 266-188. One agglomeration technique suitable for use in thepresent invention consists of mixing the alumina, carbon powder and apitch binder at elevated temperatures in a mixer or kneader andcompacting the green mix to form shaped briquettes on a roll typebriquetting press. Preferred briquettes are, for example, isometriccylinders, balls or pillow-shaped briquettes which can be formed inlarge numbers on ringroll mills or bench presses. The size of thebriquettes is between about 1 and 10 cm in diameter (for ball orcylinder shaped articles), or 1-10 cm height or thickness (forbriquettes in rectangular or flattened shapes). The preferred briquettedimensional range is between about 3 and 6 mm (diameter, height, orthickness depending on shape). The compacted and formed mixture ofaluminum oxide and carbon binder is then surrounded with a shell ofexclusively carbon. This shell is developed from a likewise plasticallyformable mass, as prepared for example from petroleum coke powder andpitch. The briquette of the charging material can be compared with thestructure of a hazelnut, the mixture of aluminum oxide and carbon binderserving as the core and low-ash coke dust and pitch serving as theshell. This carbon shell can be pressed around or rolled on, as inpelletizing.

The green briquettes are then subjected to a coking process. In thisprocess, the pitch inside the shell and the carbon binder inside thecore are transformed into solid carbon or coke. The coked pitch insidethe shell has to fulfill the task of a good binder coke since highstrength requirements are made on the carbon shell. The carbon shellmust withstand certain compression, impact and abrasion stresses in thesubsequent stages of the process. The wall thickness of the shell isexactly dimensioned so that the outer cover of the core mixture will beequal to the stresses to which the briquette is subjected. The exactwall thickness that is selected is varied and depends on the nature ofthe stresses, impact and compression that is present in the subsequentprocess steps, the thickness being greater with increasing stress orabrasion. The coked carbon shell serves the additional tasks ofconducting the electric current and as an electrical resistance elementfor heating of the agglomerate.

Generally speaking the carbon or graphite capsule or jacket serves as ahousing for the agglomerate during the reducing reaction. The amount ofcarbon required for reduction of the metal oxide and formation of thecarbide is provided in the agglomerate.

In the process, the briquettes are heated by electrical resistanceheating. According to the invention, in a compact mass of briquettes(i.e. one in which a mass of briquettes are in contact with one another)the electric current flows mainly from briquette to briquette over thecarbon shells. In the reduction of the aluminum oxide with carbon toaluminum carbide the carbon shell remains as a sturdy casing. The carbonshell of the briquettes insure that the electrical resistance conditionsand heat supply remain largely uninfluenced by changes in the coreduring reduction. The reducing gas leaves the core via the naturallyexisting porous channels in the carbon shell. The carbon shell alsoserves as a small transporting vessel for the aluminum carbide producedby the reaction.

The strength of the core mixture of oxide and coke is of minorimportance. The coking residue of the carbon binder in the core mixtureas reducing carbon for the aluminum oxide and is sufficient for theinduction to aluminum carbide. In the coked core mixture, for example,the coke content and/or the reducing carbon amounts to about 30-35%. Theinvention will be further explained with reference to the drawing inwhich,

FIG. 1 depicts a cross sectional view of a ball shaped briquette used asa charging material in the present invention;

FIG. 2 depicts a cross sectional view of a combined calcining andreduction furnace according to the invention;

FIG. 3 illustrates a cross sectional view of an extraction reactoraccording to the present invention; and

FIG. 4 depicts a flow diagram of the process of the invention.

FIG. 1 shows a cross sectional view of a ball-shaped briquette of thecharging material. In this drawing, a denotes the carbon shell and bdenotes the core mixture of aluminum oxide and carbon. Prior to beingplaced in the reducing furnace, the green briquettes must be coked orcalcined. The calcining takes place either separately from the reducingfurnace in shaft, tunnel or rotary tubular furnaces up to temperaturesof about 800-1000 degrees centigrade or directly in a preliminary stageof the reducing furnace. Since the briquettes must all be heated to thereducing temperature a combined furnace unit for both calcining andreducing will save energy.

FIG. 2 illustrates a combined calcining and reducing furnace. Thebriquettes are heated and calcined in the indirectly heated (e.g., bymeans of gas) shaft section I of the furnace. The reduction of thealuminum oxide takes place by direct electrical resistance heating infurnace section II. The reduced material is removed in furnace sectionIII.

The charging material from the briquettes is introduced via a hopper 1and entry port 2 into the vertical muffle space 3.

The inside wall 7 of the vertical muffle 3 is constructed of silicabricks or fire clay bricks having an alumina content greater than 60%.The flues 8 have exterior refractory walls 9. The furnace has anexterior shell 10, preferably of steel. During the descent and heatingof the charging material in the furnace, the volatile pitch componentsfrom the shells and the binder of the core mixture are expelled if greenbriquettes are used. Together with the reducing gas from furnace sectionII, which mainly consists of carbon monoxide, the volatile componentsleave muffle space 3 through skylights 11. A part of the gaseous mixtureof volatile components and reducing gas is led into flues 8 and burnedwith preheated air, which is conveyed via channels 13. The hot wastegases leave shaft section I via channel 12. From here, the hot wastegases are moved to a heat exchanger (recuperator) for preheating theair. The second part of the gaseous mixture is drawn off via line 14 andburned, e.g., in a boiler plant or otherwise disposed of or recovered.The distribution of the gas streams into flues 8 and line 14 isregulated by valves 15. The heat of combustion of the gaseous mixture ofreducing gas and volatile components of the charge is considerablygreater than is required for the external heating of the muffle. At thelower end of the muffle space 3, the briquettes reach a temperature ofabout 1300 degrees centigrade.

From muffle space 3, the precalcined, solid briquettes of the chargemove into the reducing space 16 of furnace section II. The electriccurrent for heating of the charge is conveyed through side electrodes20/21 and central electrode 23/24. The side electrodes 20/21 arepositioned in electrically non-conductive tubes 22. The electrodes 20and 24 consist of electrographite and are screwed onto water-cooledshafts 21 and 24. In the zone of the electrodes, the current, whetheralternating or direct current, flows over the bulk formed by thebriquettes. The current is adjusted so that temperatures of 1950 to 2050degrees centigrade are reached there and the core mixture of briquettesis converted to aluminum carbide. Furnace section II is internally linedwith carbon bricks 17. A heat-insulating layer 18 of ceramic refractorymaterial is disposed between the carbon brick structure 17 and thewater-cooled outer jacket 19.

The core-reduced, aluminum carbide-containing briquettes are removedwhile hot via the refractory-lined channels 25 in furnace section IIIand then via vibration chutes 27. The channels 25 are lined with aceramic material 26 such as mullite or alumina bricks. The dischargetemperature lies at about 1500-1600 degrees centigrade. In place ofvibration chutes, rotary tables or screw conveyors can be used. Theremoval and transfer of the fully reduced charging material tocontainers or directly to an interconnected extraction reactor (e.g., asin FIG. 3) is effected under the exclusion of air.

The process of carbide production described above for aluminum cansimilarly be employed for other carbide-forming metals that are hard toreduce such as, for example, boron, silicon, titanium, zirconium,tantalum, niobium, molybdenum, tungsten or uranium for which similardifficulties arise in arc furnace reduction as in the reduction of Al₂O₃. The advantage of the process according to the invention liesprimarily in the fact that the carbides of the aforenamed metals can beobtained in continuous operation. The processes known hitherto forobtaining these metal carbides operate in a discontinuous manner. It islikewise possible to obtain high-melting borides, e.g., titanium borideor zirconium boride, directly from the respective oxides in a continuousoperation.

The aluminum carbide obtained from the carbothermic aluminum oxidereduction in carbon capsules constitutes an intermediate product fromwhich the aluminum is to be extracted. The extraction of the aluminumtakes place with gaseous aluminum fluoride, AlF₃, at temperatures above1100 degrees centigrade. (AlF₃ sublimes at about 1100 degreescentigrade). It is known that aluminum fluoride reacts at hightemperatures with aluminum and forms gaseous aluminum subfluoride, AlF,which with temperature decrease disproportionates again to aluminum andaluminum fluoride. The transport reaction via the aluminum subfluoridecan be used for the extraction of aluminum from its carbide.Temperatures of 1500 to 1600 degrees centigrade are required for thereaction of aluminum carbide with aluminum fluoride to form aluminumsubfluoride.

The process according to the invention provides for maintaining atemperature difference, in a closed reactor space, between the reactionsites for formation and disproportionation of aluminum subfluoride,which will allow an automatic continuous process of aluminum extraction.

The extraction reactor and its manner of operation will now be explainedmore fully with reference to FIG. 3. The extraction reactor consists ofa central, cylindrical reaction space 30 and an annular space 31. Thecentral reaction space 30 is formed by a thick-walled graphite tube 32with oblique windows 33. Graphite tube 32, because of its large size, iscomposed of individual rings that are inserted into one another. At thetop and bottom, graphite tube 32 ends in connection rings 34 made ofgraphite into which current-carrying bolts 35 are screwed. The contractpressure between graphite tube 32 and connection rings 34 is assured bypressure springs 36. The graphite tube 32 is heated in a resistanceprocess with electric current. At its outer circumference, the reactoris lined with Al₂ O₃ -containing bricks 37. The bricks 37 contain a highquantity (more than 70%) of alumina and may be at least mullite or atmaximum pure corundum bricks.

The aluminum carbide-containing material is introduced via a sealablechamber 38 into the central reaction space 30 at an entry temperature ofover 1100 degrees centigrade. The carbon material remaining after thealuminum extraction is discharged through cooling space 44 and then viathe discharge tray 39 with scraper 40. The central reaction space 30 isalso charged with aluminum fluoride when starting the reactor with thealuminum carbide-containing material. The interior temperatures of about1500-1600 degrees centigrade within reaction space 30 allow the aluminumfluoride to vaporize. The aluminum fluoride condenses in a layer 41 onreactor wall 37. The aluminum fluoride layer 41 brings about anadditional heat insulation, its thickness increasing with a sufficientAlF₃ supply until the temperature on the inner surface of layer 41 risesto above 1100 degrees centigrade and no further AlF₃ is condensed. Thereis thus formed an AlF₃ atmosphere inside the reactor. In the centralreaction space 30, the AlF₃ reacts at temperatures between 1500 and 1600degrees centigrade with the aluminum carbide to form AlF which thendiffuses through to the cooler reactor wall 37 and to the condensed AlF₃layer 41 respectively, where it disproportionates again to aluminum andgaseous AlF₃. In this way, aluminum is constantly transported from theinner space of the graphite tube through windows 33 and annular space 31to the surrounding wall 41/37. The aluminum deposited there descendsdown the wall, collects at the bottom 42 of annular space 31 and isdrawn off discontinuously through a taphole 43 or continuously through asiphon (not shown in the Figure).

The temperature of the charging material to be extracted must be higherin the transition zone from chamber 38 to central reaction space 30,than the condensation temperature at the surface of layer 41 so that noaluminum deposits on the charge. The charging material is introducedinto the extraction reactor through an aperture 46 and a slide closure45 of refractory material which is opened during charging. This chargingdevice can also be of a different design, e.g., a bell or flap closure.The discharge temperature of the carbon material remaining after theextraction should likewise be higher than the surface temperature oflayer 41. The carbon residues in the upper section of cooling space 44provide an effective heat insulation in downward direction and thusprevent disproportionation of AlF there.

The extraction reactor can have a circular or rectangular cross section.Also several extraction units can be combined into a set.

As mentioned before, the aluminum oxide-carbon mixture is enclosed witha sturdy carbon shell and the Al₂ O₃ --C core reduced to aluminumcarbide in a reducing furnace (FIG. 2). Prior to entry into theextraction reactor, the carbon shell is preferably broken up or blastedto allow the AlF₃ easier access to the aluminum carbide. Basically, thecarbon material discharged from the extraction reactor is returned tothe process cycle, i.e., used again for preparing Al₂ O₃ -carbon mixtureor encasing in carbon.

In a further preferred embodiment of the invention, one may also useprefabricated carbon or graphite vessels for the aluminum oxide-carbonbinder mixture. It is not necessary that the carbon containers enclosethe Al₂ O₃ --C mixture all around. Preferred container configurationsare cylindrical pots or sleeves that are open on one or both sides.Prefabricated carbon and graphite containers are conveniently usedrepeatedly in the process cycle until they must be replaced due to wearor breakage.

The use of prefabricated containers is more fully explained in thefollowing example:

Using an extruder device, pipes are extruded from a suitable carbonmaterial (e.g., the same base material used for the shells or casings ofthe agglomerated particles--petroleum coke filler material and electrodepitch) and then baked at 1200 degrees centigrade in an annular bakingfurnace or fully graphitized. The carbon tubes are cut into uniformsection, i.e., sleeves or rings. The prefabricated carbon sleeves arefilled with a plastic mixture of aluminum oxide, petroleum coke powderand a tar base binder material. To give the mixture inside the carbonsleeve a better grip the sleeves can have inward directed teeth formedalong with the extrusions. The filled sleeves serve as charging materialboth for the reducing furnace according to FIG. 2 and the extractionreactor according to FIG. 3. In shaft section I of the reducing furnace,the Al₂ O₃ --C binder mix is coked, and in the reducing section II themass is reduced to aluminum carbide. The carbon sleeve is the supportingshell as well as the resistance heating element for the Al₂ O₃ 3--C massinside it. In the shaft section, the Al₂ O₃ --C binder mixtures loses,for example, 5-20% of its weight and shrinks as a result. During thereduction to aluminum carbide, the coked Al₂ O₃ --C mass undergoes afurther loss in weight of about 55%. Here too, a slight shrinkageoccurs. The brown to brown-black aluminum carbide forms a porous,spongy, compressible structure. In the extraction reactor, the hollowspaces between the supporting sleeves of carbon make possible aneffective diffusion exchange, i.e., transportation of gaseous AlF₃ tothe aluminum carbide and carrying away of gaseous AlF.

A slight addition of calcium fluoride and/or magnesium fluoride to thealuminum fluoride in the extraction reactor of up to about 5 weight %has proven to be effective for promoting coagulation of separatedaluminum droplets.

The following example provides further details on the thermal extractionof aluminum by the process of invention.

100 parts of powdery alumina with a maximum particle size of 100micrometers is thoroughly mixed with 24 parts of pulverized petroleumcoke and 46 parts of electrode pitch having a softening point of 70degrees centigrade. The mixing procedure is carried out at a temperatureof about 180 degrees centigrade in a heated mixer. The green mixture,still in the warm state, is pressed into short cylindrical graphitetubes on a die press. A specific pressure of 5 N/mm2 is applied forcompacting the mixture in the tube. The outer diameter of the tubes is50 mm, the wall thickness 4 mm and the tube length 100 mm. One tubecontains about 110 cubic centimeters of the green mix corresponding to aquantity of approximately 200 grams.

The graphite tubes which are filled with the mixture of alumina,petroleum coke powder, and pitch are charged into the muffle 3 of areduction furnace as illustrated in FIG. 2. The circular muffle has aninside diameter of 1.5 m and a height of 8 meters. It is heated by meansof gas from the outside. The tubes move slowly down the muffle and areheated up to about 1000 degrees centigrade at the lower outlet of themuffle.

During the heating-up operation, the pitch binder is coked, giving acoke-residue of approximately 65%. Thus, 30 parts by weight of cokeremain in the mixture from the original 46 parts of pitch, or in otherwords, the coked mixture is composed of about approximately 65 parts byweight of alumina and 35 parts by weight of carbon.

In the lower section II of the reduction furnace the graphite tubes arebrought to a temperature in the range of 1950 to 2000 degreescentigrade. An electric current of 50 kA at a voltage of 60 V is passedthrough the packing of the graphite tubes serving as a resistormaterial. The alumina-carbon mixture in the graphite tubes is convertedto aluminum carbide. The average residence time of the graphite tubes inthe reduction furnace from charging to discharging amounts to about 12hours.

After the carbide-bearing graphite tubes are discharged from thereduction furnace, they are transferred to an extracting reactor as inFIG. III. A cooling down of the reactor feed below 1200 degreescentigrade is avoided. For this reason, the carbide bearing tubes areconveyed in closed vibrating tubular carriers to the feeding device ofthe reactor. The central graphite muffle 30 in the reactor provides theheat necessary for the extraction process. The internal diameter of thegrapite muffle is 1.2 meters; its height is 6 meters. Extractionproceeds faster than reduction. Thus, the residence time of thecarbide-containing graphite tubes in the graphite muffle is only 3.5 to4 hours. The extraction is a self-maintaining circulating processbetween two temperatures. The temperature in the graphite muffle is keptclose to 1500 degrees centigrade, the surface temperature of the outsidewall which is covered with aluminum fluoride slightly above 1100 degreescentigrade. Aluminum is produced at a rate of approximately 300 kg perhour and is tapped at the bottom of the reactor at a temperature of 1100degrees centigrade. The carbon which is set free by the decomposition ofthe carbide is removed from the graphite tubes and recycled. Also, thegraphite tubes are used again as containers for a new cycle.

The flow diagram in FIG. 4 provides a general overview of the varioussteps of the process according to the present invention.

The aforementioned metal carbides of boron, silicon, titanium,zirconium, tantalum, niobium, molybdenum, tungsten or uranium are hardmaterials, that are widely used in industry. For the production of hardmetals, carbides of titanium, tantalum and tungsten are preferably used.Titanium carbide, TiC, is for example produced according to the processof the present invention in that a mixture of titanium dioxide, carbonblack and pitch is prepared so that when coked, 3 mols carbon exist permol TiO₂. The mass of titanium dioxide, carbon black and pitch ispressed into graphite sleeves and the so obtained briquettes chargedinto the calcining and reducing furnace according to the invention. Thereduction to titanium carbide takes place at temperatures of around 2000to 2500 degrees centigrade. Maintaining of narrowly limited temperatureranges is here not of the same importance as in the preparation ofaluminum carbide. The reduction of titanium dioxide can, for example, becoupled with that of tungsten trioxide to directly obtain co-carbides ofTiC and WC, as needed for hard metal production.

It is also possible to introduce hydrogen or nitrogen into the reducingfurnace from the direction of the discharge end of the process either toimprove by means of hydrogen the reduction conditions or to formcarbonitrides in the case of nitrogen.

What is claimed is:
 1. An apparatus for the thermal production of metalcarbides comprising a vertical furnace including an upper inlet sectionand a lower bifurcated discharge section, said upper and lower sectionsbeing in open communication with one another;an indirectly heatedvertical muffle in said upper section for calcining material introducedto said upper inlet section, and at least two electrodes extending intothe lower section of said furnace and mounted on the sidewalls thereof,and a third electrode mounted in a discharge area in the center of saidlower section, said lower section capable of reducing material calcinedin said upper section.
 2. The apparatus of claim 1 wherein said lowersection comprises a conical opening in which said discharge area isformed.
 3. The apparatus of claim 2 further comprising a central,cylindrical reaction space.
 4. Apparatus as defined in claim 1 whereinsaid furnace includes an exterior wall containing a water-cooled jacket.5. Apparatus as defined in claim 4 further including a series of hollowpassage ways for conducting heated gaseous components in the walls ofsaid upper furnace section.
 6. Apparatus according to claim 4 whereinsaid conical lower section comprises discharge channels lined with arefractory material, said discharge channels communicating with at leastone vibratory passageway for conducting materials discharged from saidfurnace to a container or an interconnected extraction reactor in theabsence of air.